1. Field of the Invention
The present invention relates to an exhaust gas particulate purifying apparatus provided with a filter to purify exhaust gas by separating and collecting particulates contained in the exhaust gas discharged from an internal combustion engine such as a diesel engine.
2. Description of the Related Art
For example, a large amount of combustible particulates such as carbon particles are contained in the exhaust gas of a diesel engine. Therefore, a particulate filter (referred to as a filter, hereinafter) to collect the particulates is attached to an exhaust gas system of the diesel engine.
For example, this filter is made of a ceramic which is heat-resistant and porous so as to provide the property of gas permeability. As the operating time of an engine is increases, an amount of particulates accumulated in the filter also increases, and then the ventilation resistance gradually increases. As a result, output of the engine is reduced. For this reason, it is necessary to periodically regenerate the filter in accordance with the amount of collected particulates.
A process of regenerating the filter starts when the accumulated particulates are heated by a heating means to a temperature not less than the ignition temperature (approximately 650.degree.) so as to ignite and burn the accumulated particulates. An electric heater or a kerosine burner may be used as the heating means. In FIG. 1, an example of the exhaust gas particulate purifying apparatus is shown, in which an electric heater 21 is attached to a filter 20. The filter 20 is provided with a bypass 9. Accordingly, in the regenerating process, all exhaust gas flows through the bypass 9 by the action of a changeover valve 3 as shown in the drawing.
Air used for regeneration processing is supplied to the filter 20 from one end of the filter 20. In this example, the air used for regeneration processing is supplied in the same direction as that of the flow of exhaust gas in the case of particulate collection. When the electric heater 21 which is disposed along an upstream end surface of the filter 20 with respect to the regenerating air flow, or embedded inside the end surface, is energized with current the particulates accumulated in the filter 20 are heated and ignited from the upstream side, so that they are burnt and incinerated in the air sent from an air pump 12.
In this connection, according to the experiments made by the inventors, an accumulated particulate amount and a peak combustion temperature were strongly correlated. That is, the more accumulated particulates were available, the higher the peak combustion temperature, and it was found that the filter cracked when the temperature was raised to a value exceeding 900.degree. C. FIG. 3 is a view showing the inside of a cracked filter. As shown in the drawing, cracks C were caused at a position in the filter separated from the end surfaces of the heater by 30 mm in such a manner that the cracks were directed from the outer circumference to the inside of the filter. The diameter of the filter was 140 mm, the length was 130 mm, and the volume was 2 l.
As illustrated in FIG. 4, the cracks C were caused in the following manner:
(a) First, when the heater is energized with current, a portion close to the heater is heated to a high temperature and expanded. PA1 (b) After that, the particulates located close the heater are ignited and a combustion region A is formed. Therefore, a portion of the filter on the downstream side with respect to the heater end surface is expanded. PA1 (c) After that, the current supply to the heater is stopped. Then, the combustion region A is moved to the downstream side by the influence of an air flow for regenerating, so that a portion of the filter close to the combustion region A is expanded. However, a portion of the filter close to the heater is cooled by the regenerating air at normal temperature, so that the portion shrinks. Due to the expansion and shrinkage, an extremely high thermal stress is generated in an arrowed portion B of the filter, so that the filter is imaged.
The inventors found the above phenomenon. The more the accumulated particulate amount was increased, the more the filter was cracked. The reason is described as follows: The more accumulated particulates are available the higher the calorific value, the higher the temperature is raised. Accordingly, the amount of expansion in the combustion region A is large. On the other hand, air cooling is always conducted to the same degree. Therefore, the temperature gradient between the cooled portion and the combustion region A is increased. As a result, a high thermal stress is generated, and the filter is imaged.
In FIG. 5, it is shown that a relationship exists between the accumulated particulate amount and the regenerating ratio, and also that a relationship exists between the peak combustion temperature and the regenerating ratio in the case where the temperature immediately before the filter is regenerated is approximately the same as the room temperature. The temperature immediately before the filter is regenerated will be referred to as "filter preheating temperature", hereinafter. In this example, the filter is made of cordierite the volume of which is 2 l.
As a result, the following was found:
In order to prevent the occurrence of thermal damages such as cracks on the filter, it is necessary to maintain the peak temperature at less than about 900.degree. C., so that an amount of particulates accumulated in the filter must be not more than about 7 g/l. On the other hand, the regenerating ratio, which represents how much accumulated particulate is burnt and removed, is strongly correlated with the accumulated particulate amount, and the higher the accumulated particulate amount is, the higher the regenerating ratio. When unburnt particulates are left in the filter in a regenerating process, a pressure loss condition continues over a long period of time in the filter, which is disadvantageous from the view points of fuel consumption and of output from the engine. Further, in the next regenerating process, the unburnt particulates affect the regenerating operation, and there is a high possibility that a malfunction is caused and that the temperature is raised to an abnormally high value. For this reason, the amount of unburnt particulates are preferably small, that is, the regenerating ratio is preferably high. Consequently, in order to provide a high regenerating ratio at a temperature of not more than 900.degree. C., the amount of particulates to be regenerated must be 7 (g/l).
However, actual vehicle operating conditions vary from a high speed and heavy load operation on an uphill road, to a low speed and light load operation in a city area where traffic is congested. Therefore, the exhaust gas temperature varies greatly. Accordingly, the temperature inside the filter at a time when the regenerating processing is started, which will be referred to as "filter preheating temperature" hereinafter, varies in a range from 50.degree. C. to 700.degree. C. For this reason, it is assumed that the regenerating properties are also changed. FIG. 6 is a diagram showing the result of an experiment made by the inventors, in which the transition of temperature at a position close the heater inside the filter is illustrated in the case where the electric heater was energized with current under the condition that the filter preheating temperatures were 60.degree. C. and 400.degree. C. Usually, the heater energizing time t.sub.v is determined so that the accumulated particulates can be positively ignited even when the filter preheating temperature is low. Therefore, in the case of the curve I in which the filter preheating temperature is 60.degree. C., the temperature inside the filter is maintained to be approximately 800.degree. C. However, in the case of the curve II in which the filter preheating temperature is 400.degree. C., the temperature is approximately raised to 1000.degree. C. which exceeds the limit of occurrence of cracks.
That is, the first problem is described as follows. An extremely high thermal stress is generated between a cooled portion and a combustion area so that thermal damage is caused when the portion close to the heater end surface is cooled in the case where a current supply to the heater has been stopped. Then, the second problem is described as follows. When the filter preheating temperature is high, the heater is energized with an excessive amount of current, so that thermal damage is also caused.
FIG. 7 contains diagrams showing the result of an experiment in which the influence of filter preheating temperature was investigated in the regenerating process. In the diagrams in FIG. 7, the horizontal axis represents the preheating temperature, and the vertical axis represents the peak combustion temperature under the condition that the accumulated particulate amount is maintained at a constant value of 7 (g/l). From a preheating temperature of room temperature, the peak combustion temperature was approximately 900.degree. C. As the preheating temperature was raised, the peak combustion temperature was also raised and exceeded the temperature at which the filter was cracked.
That is, an accumulated particulate amount to be adopted as a target in the regenerating process, must be determined to be a value at which a high regenerating ratio can be provided at the lowest preheating temperature to be estimated. However, in actual vehicle operating conditions, the range of the preheating temperature is so wide that the preheating temperature is raised high. When the regenerating processing is performed under the aforesaid condition, the calorific power becomes too high. As a result, the filter is cracked as described before. For example, the aforesaid case occurs in the following manner:
Under the condition of an accumulated particulate amount which is a little smaller than a target value, a vehicle is operated at high speed under a heavy load, so that the particulates are accumulated while the filter preheating temperature is maintained high, and a regenerating processing time has arrived under the aforesaid condition.
In order to solve the above problems, when the preheating temperature is high, air for use in regenerating processing may be supplied so as to lower the temperature, and after the filter has been cooled, the regenerating processing may be conducted. However, when the aforesaid means is employed, exhaust gas containing particulates is discharged through a bypass, so that the exhaust gas emission is deteriorated.